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Optimization of the reference fields for high-energy photon and electron radiation

10.05.2006

The stability of the dose rate and radiation quality in high-energy photon and electron reference radiation fields on the PTB’s linear accelerator could be clearly improved by different modifications of the accelerator.

PTB has a linear accelerator of type Philips SL 75/20 at its disposal which generates reference radiation fields of high-energy photon and electron radiation. These reference fields are, among other things, required to realize the unit of the water absorbed dose, to investigate the response of dosemeters, to determine (radiation-quality-dependent) correction factors or to perform irradiations with defined doses for scientific purposes.

In the accelerator’s standard configuration, a drift of the dose rate of more than 30% and a variation of the radiation quality are observed when measurements or irradiations are carried out over a period of several hours. When, for example, the response of ionization chambers is measured in the water phantom, a stability of approx. 0.5% ... 1% can be achieved over a period of several hours by reference to a suitable monitor signal.

The analysis of correlations in the accelerator’s operating parameters and variation of the dose rate showed that the fluctuations observed are probably mainly due to a drift of the temperature of the acceleration length and to insufficient stabilization of the magnetron high frequency. To improve the stability of the dose rate and radiation quality of the reference radiation fields, the following modifications were, therefore, performed on the accelerator:

  1. A PID control was installed for the inlet temperature of the cooling circuit of the accelerator tube and for the flow rate of the cooling water, by which the temperature of the accelerator tube is stabilized (also in the case of changing power ratios) in a range of less than 0.1 K.
  2. The magnetron high frequency, which is regulated in a range of approx. ±30 kHz in the standard configuration of the accelerator, could be stabilized in a range of ±10 kHz by installation of a new analog PID control. In addition, the magnetron frequency can now also be regulated digitally with the aid of a laptop which allows a more exact stabilization to be achieved in the range of ±5 kHz.
  3. The standard configuration of the accelerator provides only two different high-voltage clipping stages for charging of the “pulse forming network“ (PFN). A new circuit was developed and incorporated which allows precise, adapted charging of the PFN for each of the 10 energy stages of the accelerator. In addition, this circuit is equipped with an interface to the computer so that - if and when required - digital “fine tuning“ of the dose rate generated is possible in the range of a few percent.

After these modifications, the dose rate variation generated by the accelerator amounts to less than 1% over a period of several hours; variations of the radiation quality can no longer be observed. Related to a suitable monitor signal, the response of an ionization chamber varies over a period of several hours by less than 0.1% which allows, for example, precise measurement of small corrections. An example is shown in the figure. Here, the perturbation effect of the glass cylinder used in the water calorimeter (wall thickness: 0.7 mm) was experimentally investigated in the 16 MV photon radiation field. For this purpose, the dose rate was measured several times in the water phantom using an ionization chamber with and without this glass cylinder. The measurement values stated in the figure clearly show that the dose rate is (on average) reduced by 0.07% after incorporation of the glass cylinder.

The clear improvement of the stability of high-energy reference radiation fields plays an essential role in the (very time-consuming) absolute determination of radiation quality correction factors with the aid of water calorimeters.

Figure: Influence of the glass cylinder used in the water calorimeter on the dose rate at the measuring location. The single points represent dose rate measurements with an ionization chamber NE 2561: the blue points without, the red points with glass cylinder in the phantom.